Canine gut receptors mediating pancreatic responses to luminal L-amino acids

Pancreatic protein and bicarbonate outputs were measured in dogs with pancreatic fistulas receiving intravenous secretin while individual L-amino acids or mixtures of L-amino acids were perfused into proximal intestine, Both phenylalanine and tryptophan were potent stimuli of pancreatic secretion; alanine, leucine, and valine increased protein outputs but their effects were small and not statistically significant. Six other amino acids were ineffective. Tryptophan perfused together with phenylalanine augmented responses to phenylalanine; methionine, ineffective when perfused alone, did not alter responses to phenylalanine when perfused with the latter. Responses to phenylalanine perfused at concentrations above 8 mM were dependent on load and were mediated by lengths of proximal bowel greater than 10 cm.

Canine pancreatic responses to intestinally perfused proteins and protein digests

Pancreatic bicarbonate and protein secretory responses to intestinally perfused proteins or digests of proteins were measured in dogs with chronic gastric and pancreatic fistulas when luminalpancreatic protease concentrations were reduced to undetectable levels. Protein digests were analyzed for amino acid content, and various other indirect met-ods were used to assess the composition of the digest mixtures. Of five undigested proteins, none evoked more pancreatic secretion than a control perfusion with saline. Peptic digestion of these same proteins converted four of them to polypeptides that were poten stimuli of a pancreatic juice similar in HCO3-/protein ratios to that evoked by luminal amino acids. Dialyzed peptic digests of one of the proteins, bovine serum albumin (BSA), retained potency. Likewise, digestion of BSA with endogenous or exogenous pancreatic proteases converted the protein to a stimulus about equipotent with the peptic digest, though the composition of the pancreatic digests differed markedly from that of the peptic digests. We conclude that a) luminal peptides of four or more amino acids can stimulate the pancreas and b) during protein alimentation a wide array of luminal protein products may evoke pancreatic secretion.

Canine pancreatic response to intestinally perfused oligopeptides

Although older work indicated that luminal peptides are stimulants of pancreatic secretion, these earlier experiments were performed with crude peptide mixtures containing amino acids that are also known stimulants. Furthermore, no information was provided about size or composition of stimulating peptides. For this reason, the problem was reinvestigated with commercially synthesized oligopeptides in dogs equipped with chronic gastric and pancreatic fistulas. Synthetic peptides at 30 mM concentrations were perfused into the proximal bowel when luminal pancreatic proteases were reduced to undetectable concentrations and dogs were receiving intravenous exogenous secretin infusions. Increases in pancreatic outputs of protein and bicarbonate were measured. Of the peptides tested, only glycylphenylalanine, glycyltryptophan, and phenylalanylglycine stimulated, whereas both di- and triglycine were without effect. It was concluded that some, but not all, oligopeptides in the gut lumen are stimulants of pancreatic secretion.

Similar sensitivities of pancreatic and biliary secretion to cholecystokinin plus secretin infusion

The differing sensitivities of the gallbladder and pancreas to graded doses of CCK have been claimed as important in normal digestive events. As both secretin and CCK are thought to be released in response to normal feeding, we have reexamined the sensitivities of the gallbladder and pancreas to graded increases of CCK with a constant secretin background. Under such conditions the increase in output of pancreatic trypsin and bile salt occurred simultaneously, the dose-response curves being almost superimposable. Thus, under conditions more closely approaching normal postcibal events, the previously described differing sensitivities of the pancreas and gallbladder were not observed.

99mTc-tagged chicken liver as a marker of solid food in the human stomach

Past measurement of gastric emptying of solid food in man has depended on external counting of surface-absorbed isotopes without verification that isotopic labels remain attached to solid food in the stomach. In this study chicken liver was isotopically labeled with 99mTc incorporated uniformly and intracellularly throughout the liver substance. In vitro studies showed less than 10% loss of 99mTc from liver incubated with pepsin HC1. By contrast, up to 90% of 51Cr absorbed to scrambled eggs became detached under similar conditions. In feeding experiments less than 10% of 99mTc was liberated from fed 99mTc liver, while significantly more 51Cr became detached from egg under identical intragastric conditions. We conclude that 99mTc-tagged chicken liver is an adequate marker of the rate of emptying of solid food and appears to be more reliable than 51Cr-labeled scrambled eggs from which 51Cr dissociates in the stomach.

The effect of acute hyperglycemia on gastric emptying in man

Older work in man with meals of carbohydrates in water has indicated that such meals slow gastric emptying in proportion to their osomolarities. Nevertheless, different carbohydrates have been found to have differing efficacies per milliosmole. One possibility which would explain such discrepancies among carbohydrates is that hyperglycemia induced by carbohydrate absorption itself contributes to the slowing of gastric emptying. To test this possibility, normal subjects were made acutely hyperglycemic with intravenous loads of glucose during the ingestion of various liquid test meals, and rates of gastric emptying of these meals were compared in the same subjects during periods of induced hyperglycemia with rates of gastric emptying under euglycemia conditions. Induced hyperglycemia significantly slowed the rate of emptying of meals containing fat + protein, or protein, but did not significantly alter emptying of meals containing only NaCl. It is concluded that hyperglycemia does exert some effect on gastric emptying, but that these effects of hyperglycemia are variably expressed, depending on the presence of other factors which themselves slow gastric emptying.

The potential role of protein in the absorption of fat

Stabilization of triglyceride emulsions by proteins was studied in vitro using either chemical or optical methods to measure to amount of fat emulsified under standard conditions of emulsification. Concentrations, pH, and degree of digestion of protein emulsifiers were varied to determine how such manipulations affected emulsion stabilization. Proteins from endogenous as well as exogenous sources were capable of stabilizing triglyceride emulsions, and this effect varied with protein concentration. Peptic digests of a prototype protein retained the efficacy of the undigested protein, while further digestion of the protein in pancreatic proteases decreased, but did not abolish, the emulsifying power of the protein. Emulsion stabilization by peptic digests of protein still persisted despite alterations in pH from 1 to 7. Such emulsions were capable of being hydrolyzed by pancreatic lipase and could be absorbed from the gut of bile-deficient rats. These emulsions also speed the initial rates of fat absorption from animals with an intact bile supply. We conclude that stabilization of luminal emulsions by dietary or endogenous protein may facilitate fat absorption similar to the way that detergent-stabilized emulsions have been previously shown to have these effects.

The effect of acute hyperglycemia on meal-stimulated gastric, biliary, and pancreatic secretion, and serum gastrin

The effects of hyperglycemia on pancreatic, biliary, and gastric secretory responses to meals have not been hitherto quantified in man. In the present study seven normal volunteers were fed on two occasions a 500-ml liquid test meal containing fat and protein. During one of the meals the subjects were made acutely hyperglycemic with intravenous glucose, whereas in control experiments, each subject received intravenous saline in place of glucose. A jejunal perfusion method was used to measure pancreatic outputs of trypsin and biliary outputs of bile salts for 150 min after the meal; the same method was used to quantify indirectly the amount of acid secreted by the stomach in the 150-min period. Serum gastrins were also measured basally and at intervals after the meal. Hyperglycemia suppressed serum gastrin, gastric acid production, trypsin secretion, and bile salt output in response to the test meal.